Reliquefaction of boil off gas

ABSTRACT

A method is provided for reliquefying a part of the gas boiling off from a ship&#39;&#39;s cargo of liquefied natural gas and normally used to fuel the ship&#39;&#39;s motor. The method comprises separating a portion of the boil-off gas stream that has been compressed for use as fuel, further compressing the separated portion, condensing the further compressed material and recovering the liquid fraction from the condensate. The extent of reliquefaction is increased by injecting liquefied natural gas from the cargo into the boil-off gas before condensing the latter and/or expanding a portion of the condensed material and recycling it.

United States Patent 1191 Jones Dec. 31, 1974 [5 RELIQUEFACTION OF BOIL OFF GAS 3,229,472 1/1966 Beers 62/50 x 3,0,0 2167 B ..621 [76] Inventor: James Cranboume 3,362,924 10i1 73 1:555 62/5 1 x Close, Ashton-under-Lyne, Lancashlre Engand Primary ExaminerMeyer Perlin [22] Filed: July 20, 1973 Assistant Examiner-R0nald C. Capossela pp No: 381,235 Attorney, Agent, or Firm-Browdy and Neimark [57] ABSTRACT [30] Foreign Application Priority Data A method is provided for reliquefying a part of the gas June 27, 1973 Great Britain 30455/73 boiling Off from a P cargo of liquefied natural gas and normally used to fuel the ships motor. The [52] US. Cl 60/651, 6( )/67 1, 62/7, method comprises separating a portion of the boil-off 62/50, 62/54, 62/240 gas stream that has been compressed for use as fuel, [51] Int. Cl. F0lk 25/08 further Compressing the Separated portion, condensing [58] Field of Search. 62/7, 50, 51, 54,240, the r h r mpressed material and recovering the 62/52; 60/39.4.6, 651, 671 liquid fraction from the condensate. The extent of reliquefaction is increased by injecting liquefied natu- [56] Referenc Cited ral gas from the cargo into the boil-off gas before con- UNITED STATES PATENTS densing the latter and/0r expanding a portion of the 1,808,439 6/l93l Serriades 60/671 condensed materal and recyclmg 2,940,268 6/1960 Morrison 62/7 12 Claims, 1 Drawing Figure RELIQUEFACTION OF BOIL OFF GAS This invention relates to the reliquefaction of boil-off gas from cargoes of liquefied natural gas (LNG).

In tankers carrying liquefied natural gas (LNG) a portion of the cargo generally between 0.1 and 0.25 percent per day evaporates as a result of imperfect insulation. Hitherto, this boil-off gas has normally been used to fire the ship's boilers. In view of the rising price of natural gas, however, it is now becoming more economical to reliquefy the whole or a part of the boil-off gas and return it to the cargo tanks and to use cheaper fuel oil for firing the boilers. It is therefore desirable to provide means for reliquefying the boil-off gas which can be installed on existing tankers with the minimum of inconvenience and preferably which can make use of existing equipment aboard the tankers.

Tankers carrying LNG and utilizing the boil-off gas to fuel the boilers generally include in their equipment one or more compressors for compressing the boiloff gas for feeding to the boilers, and a heater for raising the temperature of the boil-off gas from the compressors to ambient temperature. The heater generally uses steam as the heating medium and in some cases the heating medium may be water.

The present invention provides a method of reliquefying a part, typically 30 percent, of the boil-off gas, which permits the utilization of these existing ship board facilities and in which the additional equipment required can be installed very simply.

According to the present invention, the method of reliquefying a part of the boil-off gas comprises separating a portion of the boil-off gas stream that has been compressed for feeding to the ships boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilising at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, and recovering reliquefied boil-off gas from the liquid fraction of the expanded condensate, and injecting liquefied natural gas into the boil-off prior to cooling the further compressed portion to condense it and/or recycling a portion of the condensate after expanding said portion to a pressure which is below the maximum pressure of the further compressed portion. The fraction of the expanded condensate which remains after recovering reliquefied boil-off gas from said condensate can be used to fuel the ships boilers.

The injection of the LNG and recycling of the condensed gas each assist in increasing the proportion of the boil-off gas that can be reliquefied by the method.

Preferably, all the sensible cold in the uncompressed boil-off gas between the temperature at which it boils of from the cargo and ambient temperature is used to assist in condensing the further compressed portion of boil-off gas. This may result in the boil-off gas stream which is compressed for feeding the ship's boilers being above room temperature after compression instead of below. as would normally be the case where all the boiloff gas is used to fire the ship's boilers. In this case, if the heaters which are used to heat the compressed boiloff gas for feeding to the ships boilers utilise water as the heating medium, they may conveniently be used to cool the compressed boil-off gas instead. Alternatively,

if steam heated boil-off heaters are used, these are bypassed and the compressed boil-off gas either cooled in a separate water cooler or fed directly tothe supplementary compressor without cooling.

Where liquefied natural gas is injected into the boiloff gas in accordance with the invention, the existing submerged liquid spray pumps, designed to pump the LNG to high pressure for spraying into the cargo tanks, may be used to inject the LNG into the boil-off gas and the latent heat of vaporization and the sensible cold in the LNG may be used to supplement the sensible cold in the boil-off gas from the cargo and the cold obtained by expanding the subsequent condensate, in the cooling of the further compressed portion of the boil-off gas to condense it.

The additional equipment required for the method of the invention comprises supplementary compressor means for further compressing the portion of .compressed boil-off gas separated from the stream which has been compressed for feeding to the ships boilers, prime mover means for driving this compressor means, and a cold box including the heat exchange means for cooling and condensing the further compressed steam, one or more expansion valves and gas/liquid separator for separating the liquid from the gas in the expanded condensate. Conveniently the compressor means and its prime mover can be assembled on one skid and the cold box can be assembled on another skid for installation in the ship thereby reducing the amount of shipboard work required.

The LNG may be injected into the boil-off gas at any point prior to the cooling of the further compressed portion to effect condensation. However, it is preferred to inject it only into the separated portion of the compressed boil-off gas stream because otherwise the effect of the injection is diluted through the loss of a part of it as fuel to the ships boilers. Where the supplementary compressor means is a multi-stage compressor, the LNG can be injected at any convenient stage of the multi-stage compressor. In this instance, the LNG for injection is evaporated and superheated, most suitably by heat exchange with the high pressure gas stream from the supplementary compressor means, before mixing with the separated boil-off gas.

In the embodiment of the invention involving both the injection of the LNG and the expansion and recycling of a portion of the condensate, the recycled material may be combined with the LNG for injection into the boil-off gas. In this embodiment, therefore, the LNG together with recycle material must be injected into the separated portion before said separated portion reaches its maximum pressure as a result of its further compression.

If desired the LNG and the recycle' material may, singly or in combination, be divided into a plurality of streams which are injected into the separated portion of boil-off gas at a plurality of different pressure levels in order to reduce the energy required to compress the separated boil-off gas, LNG and recycle material.

In accordance with a preferred aspect of the invention, at least a part of the power required for further compressing the separated portion of boil-off gas may be provided by a gas engine, e.g., a gas turbine, which is fuelled by part of the boil-off gas. The fuel may be supplied from the non-liquid portion of the expanded condensate after recovery of the reliquefied boil-off gas, or from the compressed gas stream destined for the ships boilers, or by part of the gas compressed by the supplementary compressor or from a combination of two or more of these sources. However, if desired, the power may be provided by other means, e.g., one or more electric motors or an engine fuelled bya hydrocarbon fuel other than boil-off gas. Also the power may be provided by a steam turbine to which the supply of steam is that which otherwise would be used to heat the boil-off gas after its compression for feeding to the boilers, since duringthe periods when the reliquefaction plant is operating this heat will not be required.

The invention is now illustrated with reference to'one preferred embodiment thereof andwith the aid of the accompanying drawing which is a schematic flow sheet of one method of carrying out the reliquefaction. In the drawings, reference numerals 1 indicate cargo tanks containing LNG; reference numeral 2 is a'submerged liquid spray pump generally found as existing equipment aboard an LNG tanker; 3 is the boil-off gas compressor and 4 is the heater for the compressed boil-off gas, both also generally found as existing equipment aboard LNG tankers. 7 is a heat exchanger; 8, 21 and 24 are expansion valves; 9 is a gas/liquid separator; 10 is a multi-stage compressor with inter-coolers 25 and after cooler 26, and 11 is a motor for driving the compressor 10. The multi-stage compressor 10, motor 11, inter-coolers 25 and after-cooler 26 are mounted on a skid 6 while the heat exchanger 7, expansion valves 8 and 24 and separator 9 are mounted within an insulated enclosure carried on a second skid 5. This arrangement simplifies the installation of the equipment in the tanker and reduces the amount of ship-board assembly work that is required.

The cold boil-off gas, at near atmospheric pressure, passes from the tank header 12 through pipeline l3 and heatexchanger 7, in which it 'is warmed to near ambient temperature. Leaving the exchanger it passes through line 14 to the suction of the existing boil-off compressor 3 in which it is compressed to about 26 psia. The gas then either passes through the heater 4 or by-passes this heater by means of pipeline 27, valve 28 and pipeline '29. If the heating medium for the heater 4 is water, the gas'may be passed through the heater which will now act as a cooler. if however, the heating medium is, steam, the heater will be by-passed. If desired, the by-pass may contain a water cooler, not

shown. v

I Approximately 60 percent of the boil-off gas is passed through line 15 to the ships boilers. The remaining 40 percent is conducted through line 16 to the suction of the compressor 10 on skid 6. In the first stage 10a, of this compressor the pressure of the gas is raised to 55 psia.

A small stream of LNG, amounting to about 11.5 percent of the total boil-off gas, is compressed to 60 psia by the existing spray-pump 2 and conducted in pipeline 17 to the heat exchanger 7, where it isevaporated and warmed to near ambient temperature. It then passes through line .18 to the suction of the second stage 10b of the compressor 10, joining the boil-off gas delivered by the first stage. in the second and third stage thecombined stream is compressed to 700 psia and led through line 19 to heat exchanger 7, where it is completely liquefied and subcool ed to about 130K in counter-current with the other streams. it is then expanded to 44 psia in the valve 8 as a result of which it cools further to about 120K and a part evaporates.

Liquid and vapor are separated in the separator 9. The liquid is returned to the tanks through line 20, control valve 21 and line 22. The vapor, which will normally be rich in nitrogen, is returned through exchanger 7 and fed to the boilers through line 23.

It should be noted that the LNG will normally contain an appreciable quantity of nitrogen and that this nitrogen, as a result of its low boiling point compared with the other constituents of natural gas, will evaporate preferentially and form a very significant portion of the boil-off gas. For example the boil-off gas of an LNG sample containing 1 percent nitrogen may contain as much as 25 percent nitrogen. The liquefaction of the boil-off gas will therefore require cooling to a very low temperature, so that without the injection of the LNG through line 18 the liquid will be only poorly sub-cooled in exchanger 7, resulting in heavy evaporation in valve 8 and a consequent low recovery of liquid. The injection through line 18 of LNG, which has a much lower nitrogen content than the boil-off gas, enables liquefaction of the combined stream to take place in a higher range of temperatures thus allowing the liquid to be more adequately sub-cooled, thereby reducing the evaporation occurring on expansion and increasing the LNG recovery.

It will be appreciated that, if desired, the LNG may be'injected into the boil-off gas before stage 10a or before or after stage of the multi-state compressor. However alternative means of pressurising the LNG may have to be provided. It is important however, that the LNG is injected into the boil-off gas prior to the cooling of the boil-off gas in heat exchanger 7 to condense it. Equally, the LNG may be combined with the boil-off gas in pipeline 12 or pipeline 14 provided that the capacity of compressor 3 is large enough to cope with the enlarged stream.

It will also be understood that where no boil-off gas compressor is present as existing equipment, compressor 10 may be designed to provide the total required compression and may, if desired, be a single stage compressor or a multi-stage compressor having two compression stages or four or more compression stages, the three stages shown not being critical.

Improved recovery of LNG from the boil-off gas is achieved by diverting a portion of the stream of condensed gas upstream of expansion valve 8, through pipeline 30, expanding it through valve 24, and feeding it into LNG pipeline 17. Alternatively, it may be expanded toa still lower pressure and returned to line 12, 14 or 16, or expanded to the same or a higher pressure and returned as a separate stream to the inlet of the second or third stage of the compressor 10.

In another embodiment, not illustrated, the LNG pump 2 and pipelines l7 and 18 are omitted and the portion of the stream of condensed gas that is diverted through pipeline 30 may be expanded through valve 24 and returned to pipelines 12, 14 or 16 or between the first and second or second and third stages of the multistage compressor 10.

Preferably, the stream in pipeline 30 is returned to pipeline 16 or downstream thereof so that none is lost as fuel through pipeline 15. Where the recycle is addi-. tional to the LNG injection, it is only convenient to feed the expanded condensate from the expansion valve 24 .into the LNG pipeline 17, as shown, when the LNG is injected into the boil-off gas at a pressure below the maximum pressure to which it is compressed; i.e.,

at some stage up to and including the inlet to the last stage of the compressor 10.

By means of the method illustrated, about 30 percent of the boil-off gas can be reliquefied. The remainder may, as indicated, be passed to the ships boilers. Alternatively, however, a proportion of it may be used to fuel the motor 11 driving the compressor and for this purpose the motor may suitably be a gas turbine.

I claim:

1. A method of reliquefying a part of the boil-off gas from a ships cargo of liquefied natural gas, which boiloff gas has been compressed for feeding as fuel to the ships motor, said method comprising the steps of separating a portion of the boil-off gas stream that has been compressed for feeding to the ships boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilizing at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, recovering reliquefied boil-off gas from the liquid fraction of the expanded condensate, and injecting liquefied natural'gas into the boil-off gas at a point before the further compressed portion is cooled to condense it.

2. A method as claimed in claim 1 in which all the sensible cold in the uncompressed boil-off gas between the temperature at which it boils off from the cargo and ambient temperature is used in the cooling of the further compressed portion of boil-off gas to condense it.

fore the further compressed portion is cooled to condense it and a portion of the condensate is recycled after expansion to a pressure which is below the maximum pressure of the further compressed portion, wherein said portion of condensate is combined with said liquefied natural gas and the combined material is injected into the separated portion of the compressed boil-off gas stream before said separated portion reaches its maximum pressure as a result of said further compression.

5. A method as claimed in claim 1 in which at least a part of the power required for further compressing the separated portion of boil-off gas is provided by a gas engine which is fuelled by boil-off gas.

6. A method as claimed in claim 1 further comprising the step of recycling a portion of the condensate after expanding said portion to a pressure which is below the maximum pressure of the further compressed portion.

7. in a ship having a cargo tank for carrying liquefied natural gas, first gas compressor means and means for feeding boil-off gas from said tank to said first compressor means for compressing for use as fuel to the ships motor, an improvement further comprising:

i. second gas compressor means;

ii. heat exchange means;

iii. expansion valve means;

iv. gas/liquid separator means;

v. first conducting means for feeding boil-off from said cargo tank through said heat exchange means as a first coolant stream prior to feeding it to said first compressor means;

vi. second conducting means for passing a portion of the compressed boil-off gas from said first compressor means to said second compressor means for further compression;

vii. third conducting means for passing the further compressed boil-off gas from said second compressor means through said heat exchange means in indirect counter-current heat exchange relationship with said first coolant stream, said further compressed boil-off gas stream being cooled and condensed in said heat exchange means;

viii. fourth conducting means for passing the cooled and condensed gas stream from said heat exchange means through said expansion valve means to expand same and thence to said gas/liquid separator means;

ix. gas recovery means for recovering gas from said gas/liquid separator means and passing it as a second coolant stream through said heat exchange means in indirect countercurrent heat-exchange relationship with said further compressed gas stream;

x. injection means for injecting gas from said cargo tank into boil-off gas at a point before said boil-off gas is passed to said heat exchange means for cooling and condensing; and

xi. liquid recovery means for recovering liquid from said gas/liquid separator means and returning it as reliquefied boil-off gas to said cargo tank.

8. In a ship having a cargo tank for carrying liquefied natural gas, first gas compressor means and means for feeding boil-off gas from said tank to said first compressor means for compressing for use as fuel to the ships motor, an improvement further comprising:

i. second gas compressor means;

ii. heat exchange means;

iii. expansion valve means;

iv. gas/liquid separator means;

v. first conducting means for feeding boil-off gas from said cargotank through said heat exchange means as a first coolant stream prior to feeding it to said first compressor means;

vi. second conducting means for passing a portion of the compressed boil-off gas from said compressor means to said second compressor means for further compression;

vii. third conducting means for passing the further compressed boil-off gas from said compressor means through said heat exchange means in indirect counter-current heat exchange relationship with said first coolant stream, said further compressed boil-off stream being cooled and condensed in said heat exchange means;

viii. fourth conducting means'for passing the cooled and condensed gas stream from said heat exchange means through said expansion valve means to expand same and thence to said gas/liquid separator means;

ix. gas recovery means for recovering gas from said gas/liquid separator means and passing it as a second coolant stream through said heat exchange means in indirect counter-current heat-exchange relationship with said further compressed gas stream;

x. expanding means for expanding and recycling a portion of the further compressed gas stream after it has been cooled and condensed by passage through saud heat exchange means; and

xi. liquid recovery means for recovering liquid from said gas/liquid separator means and returning it as reliquefied boiloff gas to said cargo tank.

9. The improvement as in claim 8, further comprisxii. injection means for injecting liquefied gas from said cargo tank into said boil-off gas at a point before said boil-off gas is passed to said heat exchange means for cooling and condensing.

10. A method of reliquifying apart of the boil-off gas from a ship s cargo of liquified natural gas, which boiloff gas has been compressed for feeding as fuel to the ships motor, said method comprising the steps of separating a portion of the boil-off stream that has been compressed for feeding to the ships boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilizing at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, recovering reliquified boil-off gas from the liquid fraction of the expanded condensate, and recycling a portion of the condensate after expanding said portion to a pressure which is below the maximum pressure of the further compressed portion.

11. A method as claimed in claim 10 in which all the sensible cold in the uncompressed boil-off gas between the temperature at which it boils off from the cargo and ambient temperature is used in cooling of the further compressed portion of the boil-off gas to condense it.

12. A method as claimed in claim 10 in which at least a part of the power required for further compressing the separated portion of boil-off gas'is provided by a gas engine which is fuelled by boil-off gas.

3,857,245 December 31, 1974 Patent No. Dated James Kevin JONES Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, claim 8, line 47, "said compressor" should read --said first compressor-- Column 6, claim 8, line 51, "said compressor" should read --said second compressor-'- Si ned and sealed this 8th day of April 1975.

SEAL) Attest:

C. I'MRSPALL DANN RUTP C. ZwIASON Commissioner of Patents Attesting Gfficer and Trademarks 

1. A method of reliquefying a part of the boil-off gas from a ship''s cargo of liquefied natural gas, which boil-off gas has been compressed for feeding as fuel to the ship''s motor, said method comprising the steps of separating a portion of the boiloff gas streAm that has been compressed for feeding to the ship''s boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilizing at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, recovering reliquefied boil-off gas from the liquid fraction of the expanded condensate, and injecting liquefied natural gas into the boil-off gas at a point before the further compressed portion is cooled to condense it.
 2. A method as claimed in claim 1 in which all the sensible cold in the uncompressed boil-off gas between the temperature at which it boils off from the cargo and ambient temperature is used in the cooling of the further compressed portion of boil-off gas to condense it.
 3. A method as claimed in claim 1 in which the liquefied natural gas is injected into the separated portion of the compressed boil-off gas stream.
 4. The method claimed in claim 1 in which liquefied natural gas is injected into the boil-off gas at a point before the further compressed portion is cooled to condense it and a portion of the condensate is recycled after expansion to a pressure which is below the maximum pressure of the further compressed portion, wherein said portion of condensate is combined with said liquefied natural gas and the combined material is injected into the separated portion of the compressed boil-off gas stream before said separated portion reaches its maximum pressure as a result of said further compression.
 5. A method as claimed in claim 1 in which at least a part of the power required for further compressing the separated portion of boil-off gas is provided by a gas engine which is fuelled by boil-off gas.
 6. A method as claimed in claim 1 further comprising the step of recycling a portion of the condensate after expanding said portion to a pressure which is below the maximum pressure of the further compressed portion.
 7. In a ship having a cargo tank for carrying liquefied natural gas, first gas compressor means and means for feeding boil-off gas from said tank to said first compressor means for compressing for use as fuel to the ship''s motor, an improvement further comprising: i. second gas compressor means; ii. heat exchange means; iii. expansion valve means; iv. gas/liquid separator means; v. first conducting means for feeding boil-off from said cargo tank through said heat exchange means as a first coolant stream prior to feeding it to said first compressor means; vi. second conducting means for passing a portion of the compressed boil-off gas from said first compressor means to said second compressor means for further compression; vii. third conducting means for passing the further compressed boil-off gas from said second compressor means through said heat exchange means in indirect counter-current heat exchange relationship with said first coolant stream, said further compressed boil-off gas stream being cooled and condensed in said heat exchange means; viii. fourth conducting means for passing the cooled and condensed gas stream from said heat exchange means through said expansion valve means to expand same and thence to said gas/liquid separator means; ix. gas recovery means for recovering gas from said gas/liquid separator means and passing it as a second coolant stream through said heat exchange means in indirect countercurrent heat-exchange relationship with said further compressed gas stream; x. injection means for injecting gas from said cargo tank into boil-off gas at a point before said boil-off gas is passed to said heat exchange means for cooling and condensing; and xi. liquid recovery means for recovering liquid from said gas/liquid separator means and returning it as reliquefied boil-off gas to said cargo tank.
 8. In a ship having a cargo tank for carrying liquefied natural gas, first gas compressor meaNs and means for feeding boil-off gas from said tank to said first compressor means for compressing for use as fuel to the ship''s motor, an improvement further comprising: i. second gas compressor means; ii. heat exchange means; iii. expansion valve means; iv. gas/liquid separator means; v. first conducting means for feeding boil-off gas from said cargo tank through said heat exchange means as a first coolant stream prior to feeding it to said first compressor means; vi. second conducting means for passing a portion of the compressed boil-off gas from said compressor means to said second compressor means for further compression; vii. third conducting means for passing the further compressed boil-off gas from said compressor means through said heat exchange means in indirect counter-current heat exchange relationship with said first coolant stream, said further compressed boil-off stream being cooled and condensed in said heat exchange means; viii. fourth conducting means for passing the cooled and condensed gas stream from said heat exchange means through said expansion valve means to expand same and thence to said gas/liquid separator means; ix. gas recovery means for recovering gas from said gas/liquid separator means and passing it as a second coolant stream through said heat exchange means in indirect counter-current heat-exchange relationship with said further compressed gas stream; x. expanding means for expanding and recycling a portion of the further compressed gas stream after it has been cooled and condensed by passage through saud heat exchange means; and xi. liquid recovery means for recovering liquid from said gas/liquid separator means and returning it as reliquefied boil-off gas to said cargo tank.
 9. The improvement as in claim 8, further comprising: xii. injection means for injecting liquefied gas from said cargo tank into said boil-off gas at a point before said boil-off gas is passed to said heat exchange means for cooling and condensing.
 10. A method of reliquifying a part of the boil-off gas from a ship''s cargo of liquified natural gas, which boil-off gas has been compressed for feeding as fuel to the ship''s motor, said method comprising the steps of separating a portion of the boil-off stream that has been compressed for feeding to the ship''s boilers, further compressing said separated portion to a higher pressure, cooling said further compressed portion to condense it, the cooling being effected in part by utilizing at least part of the sensible cold in the boil-off gas prior to compression and in part by cold obtained by expanding the condensate, recovering reliquified boil-off gas from the liquid fraction of the expanded condensate, and recycling a portion of the condensate after expanding said portion to a pressure which is below the maximum pressure of the further compressed portion.
 11. A method as claimed in claim 10 in which all the sensible cold in the uncompressed boil-off gas between the temperature at which it boils off from the cargo and ambient temperature is used in cooling of the further compressed portion of the boil-off gas to condense it.
 12. A method as claimed in claim 10 in which at least a part of the power required for further compressing the separated portion of boil-off gas is provided by a gas engine which is fuelled by boil-off gas. 